1. Field of the Invention
The present invention relates to a steel plate reinforcing sheet and, more particularly, to a steel plate reinforcing sheet to be adhesively bonded to a steel plate used in a variety of industrial machines, such as transportation machines, to reinforce the steel plate.
2. Description of the Prior Art
In general, the steel plate for a shell of an automotive vehicle is manufactured in the form of a thin sheet having a thickness generally in the range of 0.6 mm to 0.8 mm, for reduction in weight of a vehicle body. As is known, the steel plate reinforcing sheet used for reinforcing the steel plate comprises a restricting layer and a reinforcing layer. In use, the steel plate reinforcing sheet is adhesively bonded to the inside of the steel plate and then the reinforcing layer is foamed.
A thermally curable adhesive sheet for reinforcing a thin steel plate was proposed as the steel plate reinforcing sheet. It has a structure wherein uncured adhesive layers of epoxy resin composition having an adhesiveness at room temperature and containing a thermally decomposable foaming agent are laminated in layers on a base material layer of uncured epoxy resin composition (Cf. JP Laid-open (Unexamined) Patent Publication No. 2002-283526, for example).
However, this thermally curable adhesive sheet for reinforcing the thin steel plate has the disadvantage that when cooled, distortions caused by difference in thermal expansion coefficient between the laminated adhesive layers occur in the steel plate (adherend).
On the other hand, a method of reducing those distortions was also proposed, wherein slits are formed in a diffusion restraining sheet of the reinforcing sheet in such a relation that they are extended orthogonally to a longitudinal direction of the sheet and staggered with spaced intervals in the longitudinal direction, whereby restraint of the diffusion of the thermally-curable foaming resin of the diffusion restraining sheet is relieved to prevent distortions of an outer panel caused by the expansion difference when the foaming resin is thermally foamed (Cf. JP Laid-open (Unexamined) Patent Publication No. 2000-103008, for example).
The formation of the staggered slits can provide the advantage of reducing the distortions of the steel plate, while on the other hand, it provides the disadvantage of reducing the reinforcing property of the steel plate reinforcing sheet.
Also, when the curved steel plate, such as a door panel, is reinforced by the steel plate reinforcing sheet, abrupt deformation occurs at end portions of the steel plate reinforcing sheet with respect to a direction extending along the curving direction of the steel plate after the reinforcing layer is foamed. This is because flexural rigidity of the steel plate reinforcing sheet in a direction perpendicular to the curving direction of the steel plate increases, so that the stress is concentrated on the end portions of the steel plate reinforcing sheet. Accordingly, relieving the stress from the end portions of the steel plate reinforcing sheet with respect to the direction extending along the curving direction of the steel plate is required for preventing this abrupt deformation.